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Full text of "Handbook Of Chemical Engineering - I"

PYROMETRY
413
400-300— 200-
The above table was compiled for use at the Bureau of Standards, and is only applicable to thermometers of the laboratory type used under total-immersion conditions, i.e., with bulb and stem containing the mercury column at the same temperature.
The table emphasizes the necessity for obtaining and applying corrections to the readings of thermometers especially if high accuracy is desired. The maximum errors allowed in the table represent the probable magnitude of the error that may be expected of the best grades of thermometers. This degree of accuracy cannot always be expected of cheaper grades of thermometers, or of partial-immersion thermometers.
Industrial Thermometers.—The use of the chemical or laboratory type of mercurial thermometer is restricted as the name implies to the laboratory since unprotected glass thermometers are much too frail to withstand the rough, usage of the plant and shop, and are comparatively difficult to read as well.
To meet the requirement of the plant there has been developed what is generally known as the industrial type of mercurial thermometer. As shown in Fig. 2, it is characterized by a heavy metal back and protecting tube for the bulb, large and distinct figures and graduation marks, and threaded connections for attaching the instrument firmly and quickly to some part of the apparatus.
This same general design of instruments is used for many different operations, covering ranges of temperature from —30° to 1,000°F. These instruments necessarily must be graduated and standardized for the condition of use, that is for partial immersion of the mercury column.
It will be noted from the cross-section view of the bulb and protecting case of the
industrial thermometer, that first, the bulb does not come into direct contact with the substance the temperature of which is to be measured, and secondly the bulb is surrounded by large metal parts which extend into a region which is at a different temperature from that of the bulb.
These two peculiarities in the construction of an industrial thermometer must be taken into consideration in their use. Since the bulb is not in direct contact with the heated substance the time that it requires to take up changes in temperatures is greater than that necessary for a bare bulb. This "lag" as it is called can be reduced by filling the space between the bulb and the outer wall of the casing with a good conducting medium. The most satisfactory substance to use for temperatures below 200°C-, is mercury. Experiments made at the Bureau have demonstrated that the lag of an instrument read in steam at 212°F. with mercury surrounding the bulb, was J-£ to J^ as great as when powdered graphite, oil, or air was used. Obviously mercury cannot be used at the higher temperatures, and heavy oils are usually substituted.
This "lag," however, may not be as great a source of error as the actual variance which may be noted in the readings of these thermometers when used to measure the same temperature, but under varying conditions as regards construction of the protecting case surrounding the bulb, the material into which the bulb is immersed, i.e., whether liquid, vapor or gas, the rate of flow of the material past the bulb, and the exterior conditions surrounding the protecting stem.
These variations can be understood by examination of the construction of the thermometer and an analysis of the means whereby the heat is either conducted to the
Separable Socket
.This space filled with mercury, oil or powdered graphite
FIG. 2.—Industrial thermometer.